The invention relates to a circuit breaker at least one phase of which is formed by several poles mounted in parallel.
The circuit breaker rating, i.e. the value of the rated current of the circuit breaker, is, for a case of predetermined size, determined by the choice of the poles, i.e. essentially by the dimensions of the copper parts associated to the pole.
It is desirable to be able to extend a circuit breaker range by associating circuit breakers comprising a certain number of standard poles so as to obtain, for a minimum additional cost, a circuit breaker of higher rating than that of the conventional poles which make up the circuit breaker. For this purpose, it has been proposed, in the document EP-A-0,320,412, to connect two adjacent poles of a standard circuit breaker in parallel. At least one phase of the circuit breaker is then constituted by two poles, each comprising a stationary contact extended by a contact strip protruding out from the case, a movable contact connected by a flexible conductor to a second contact strip protruding out from the frame, and an arc extinguishing chamber. One connecting strip is fixed to the contact strips of the stationary contacts of the two poles and another connecting strip is fixed to the contact strips of the movable contacts, thus achieving twinning of the two poles.
Experience however shows that when breaking occurs under these conditions, the arcing current does not divide uniformly between the two twinned poles. Very quickly, the arcing current in fact only persists in one of the two breaking chambers. If the ultimate short-circuit breaking capacity assigned to the circuit breaker remains identical to that of the original standard circuit breaker, this phenomenon does not have any drawbacks. If on the other hand a higher breaking capacity is sought for, the arcing energy becomes too great for a single chamber. The twinned pole construction of the state of the technique therefore proves unsuitable for manufacture of a circuit breaker whose breaking capacity is higher than that of the individual circuit breakers which make it up. This is why circuit breakers with high breaking capacity of the state of the technique do not use standard chambers mounted in parallel.